1. Field of the Invention
This invention relates to the field of precise call progress detectors and more specifically to providing a means for determining the duration of input tone bursts.
2. Prior Art
In the prior art, call progress detection has been accomplished by several methods. In one method, an analog input signal is fed through a frequency discriminator (for filtering out noise). The output of the frequency discriminator is coupled to a peak to peak detector coupled to a digital timer for determining the duration of the desired pulse.
A second method provides two part discrimination of the input signal by utilizing an amplitude discriminator and a frequency discriminator. This method also uses a predetermined noise floor to set a low level detect threshold. One drawback of these methods is that the amplitude discrimination filter introduces a gradual decrease in the amplitude of the tone burst at the output of the filter, (called "ringing") at levels within the dynamic range of the detector system. As a result of this gradual decrease in output amplitude the signal level into the level detection circuit is kept above the predetermined noise floor for a duration exceeding the input tone burst duration, which causes inaccuracies in the detection time signal output of the detector. For many telephone applications, the minimum duration of pulse which this prior art system could detect is approximately 500 milliseconds. However, intelligent switching systems require that tone bursts as short as 40 milliseconds be detectable.